1. Technical Field of the Invention
This document relates to an active matrix type organic light emitting display and a driving method thereof.
2. Discussion of the Related Art
An active matrix type organic light emitting display includes a self-luminous organic light emitting diode (hereinafter, referred to as “OLED”), and is advantageous in that it has high response speed, luminous efficiency, luminance, and a large viewing angle.
An OLED is a self-luminous element having the structure as shown in FIG. 1. The OLED includes an anode, a cathode, and an organic compound layer HIL, HTL, EML, ETL, and EIL formed between the anode and the cathode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL and an electron injection layer EIL. If drive voltages are applied to the anode electrode and the cathode electrode, holes within the hole injection layer HIL and electrons within the electron transport layer ETL respectively move to the emission layer EML to form excitons. As a result, the emission layer EML emits a visible ray.
The organic light emitting display includes pixels each including an OLED which area arranged in a matrix form, and controls the luminance of the pixels according to the gray scale of video data. Each pixel includes a driving thin film transistor (TFT) for controlling the driving current flowing through the OLED in accordance with a gate-source voltage, a capacitor for keeping a gate potential of the driving TFT constant during a frame, and a switching TFT for storing a data voltage in the capacitor in response to a gate signal. The luminance of a pixel is proportional to the magnitude of the driving current that flows through the OLED.
The organic light emitting display is disadvantageous in that the driving TFTs of the pixels have different threshold voltages depending on where they are formed, due to a process deviation or the like, or the electrical properties of the driving TFTs are deteriorated due to a gate-bias stress which occurs with the elapse of driving time. To solve this problem, Korean Laid-Open Patent Publication No. 10-2005-0122699 discloses a pixel circuit of an organic light emitting display which detects, as the threshold voltage of a driving TFT, a gate-source voltage at which a drain-source current becomes sufficiently small by a diode-connecting the driving TFT, and compensates a data voltage by the detected threshold voltage. The pixel circuit uses a light emission control TFT serially connected between the driving TFT and an OLED in order to turn off light emission of the OLED upon detecting the threshold voltage of the driving TFT.
However, the conventional pixel circuit of an organic light emitting display is problematic in that its capability of compensating for the threshold voltage of the driving TFT is low and some TFTs show low reliability due to the following reasons.
First, when detecting the threshold voltage of a driving TFT of a diode structure, a gate-drain voltage becomes “0V”, and thus a minimum threshold voltage (for n-type) or maximum detectable threshold voltage (for p-type) is “0V”. Therefore, according to a conventional method for detecting the threshold voltage of a driving TFT by diode connection, a pixel circuit using a n-type TFT can detect the threshold voltage of the driving TFT only when the threshold voltage of the driving TFT has a positive value, and a pixel circuit using a p-type TFT can detect the threshold voltage of the driving TFT only when the threshold voltage of the driving TFT has a negative value. In other words, a conventional method for compensating a threshold voltage cannot be applied if the threshold voltage of the driving TFT in the pixel circuit using a p-type TFT has a negative value, and also cannot be applied if the threshold voltage of the driving TFT in the pixel circuit using an n-type TFT has a positive value.
Second, a parasitic capacitance exists between a TFT of a pixel circuit and a signal line. The parasitic capacitance causes a kick-back voltage when a gate signal applied to the TFT is turned off. If the kick-back voltage is high, a detected threshold voltage cannot be properly maintained but is distorted, thus decreasing the accuracy of compensation. To increase the accuracy of threshold voltage compensation, the gate and source voltages of the driving TFT need to be increased further when detecting a threshold voltage, by taking distortion in subsequent steps into consideration. However, the conventional method for threshold voltage compensation cannot improve the accuracy of compensation because a fixed potential is applied to the gate of the driving TFT.
Third, the light emission control TFT serially connected between the driving TFT and the OLED is turned off in a period during which threshold voltage sensing and data programming are performed, and turned on in a period during which light emission occurs. Assuming that the period during which threshold voltage sensing and data programming are performed is a first period, and the period during which light emission occurs is a second period, a proportion that the second period occupies in one frame is much larger than that of the first period. Since the light emission control TFT in the pixel circuit is kept turned on during the entire emission period, the reliability of the light emission control TFT is lowered due to a deterioration caused by a gate-bias stress.